Lacustrine groundwater discharge (LGD) can play an important role in water and contaminant mass balance of lakes. Dongting Lake is the second largest fresh lake in China which is connected to Yangtze River and has quite prominent ecological status and function within Yangtze River basin. However, the effect of groundwater discharge on the balance of water and contaminant in Dongting Lake has long been overlooked. This study estimated the groundwater discharge and associated contaminants input into Dongting Lake during the dry season using multiple tracers (²²²Rn, ¹⁸O, Cl⁻). After sensitivity analysis of different models, it is found that the result of ²²²Rn mass balance model is the most reliable. Based on the ²²²Rn mass balance model, the groundwater discharge rate is estimated to be 73.94 mm/d and the contribution of LGD to water balance is 10.94%. As the main nutrient components, NH₃-N, P and Si from groundwater input account for 23.65%, 5.12% and 30.15% % of the total input, respectively. As the main heavy metal components, Fe, Mn and As from groundwater input all account for more than 50% of the total input. Although the LGD rate is relatively small, the contaminant input from LGD is significant enough, which may be a potential threat to the ecological stability of Dongting Lake. In this study, the mass balance models of multiple tracers were integrated to understand the role of groundwater in maintaining the water balance and pollution status of Dongting Lake, which has certain reference significance for the LGD study in plain lakes or reservoirs with complex water systems in humid regions.